The degree of donor cell engraftment following hematopoietic stem cell (HSC) transplantation is determined by the level of host stem cell competition. This competition is negligible after HSC-toxic effects of myeloablative conditioning, whereas the large number of host HSC remaining after non-myeloablative conditioning limits the success of non-ablative HSC transplantation (SCT). Increasing the number of transplanted HSC reportedly counteracts large reserves of host HSC, but excess donor HSC are not universally available for many patients or in certain types of SCT. The hypothesis of this proposal is that the overall engraftment potential of limited numbers of HSC can be increased by interventions at several different fronts in the SCT process, including modulation of the graft, conditioning regimens of the host, and exploitation of intrinsic host parameters. These interventions are aimed to increase hematopoietic potential, homing, survival and proliferation of transplanted HSC. The following three specific aims will be pursued: 1. Investigate how the graft itself can be manipulated prior to transplantation to increase its engraftment potential. 2. Examine how different host conditioning regimens impact homing, survival, proliferation, and subsequent engraftment potential of transplanted hematopoietic stem cells. 3. Identify and exploit intrinsic host parameters to increase overall engraftment potential of transplanted hematopoietic stem cells. A multi-factorial scheme utilizing sophisticated techniques and in vivo function to identify HSC will be employed to investigate these interventions using a murine SCT model. Our long-term goal is to understand the complex mechanisms by which transplanted HSC home to, survive within, and commence proliferation and self-renewal processes in host BM, and to manipulate these processes to increase engraftment potential of limited numbers of donor HSC. The significance of this work is the likelihood that it will provide new protocols in SCT for the ultimate benefit of patients receiving ex vivo manipulated grafts such as those processed for gene therapy purposes, or patients ineligible for myeloablative conditioning regimens or other protocols where the low ratio of donor to host HSC will likely lead to low or failed engraftment.